Iron Doped Titania/Polyaniline Composite: An Efficient Electrocatalyst for Hydrogen Evolution Reaction in Acidic Medium

Suman Lahkar , Richa Brahma , Swapan Kumar Dolui ^*

School of Science, Department of Chemical Science, Tezpur University, Napaam, Tezpur, India

* Correspondence: Swapan Kumar Dolui

Academic Editor: Diogo M. F. Santos

Special Issue: Hydrogen & Fuel Cells

Received: September 15, 2022 | Accepted: December 19, 2022 | Published: January 04, 2023

Catalysis Research 2023, Volume 3, Issue 1, doi:10.21926/cr.2301002

Recommended citation: Lahkar S, Brahma R, Dolui SK. Iron Doped Titania/Polyaniline Composite: An Efficient Electrocatalyst for Hydrogen Evolution Reaction in Acidic Medium. Catalysis Research 2023; 3(1) :002; doi:10.21926/cr.2301002.

© 2023 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

The ever-rising global demand for energy and awareness of environmental issues related to fossil fuel drifted the research interest to search for clean and sustainable energy resources for substitution. In modern society, energy and the environment are the two key issues necessary for developing an eco-friendly world. Among the alternate fuel sources, including methanol, ethanol, propane, hydrogen, biodiesel, etc., hydrogen production has been considered the best crucial substitute for fossil fuel due to its highest possible energy density [1], sustainability, zero greenhouse gas emission property [2], and most importantly, zero by-product. But commercial hydrogen production is pioneered by fossil fuels, which is counter-productive to our cause since steam reforming of natural gas releases massive amounts of greenhouse gases into the atmosphere. A serene way to produce hydrogen gas is via electrochemical water splitting, where hydrogen gas is released by a process called the hydrogen evolution reaction (HER), promoted by an electrocatalyst [3,4]. The efficiency of this reaction lies in its full-time operation regardless of weather conditions. For decades until now, Pt has been considered the most effective HER catalyst due to its highest cathodic current density. But less abundance and high cost of Pt halt the broad utilization preventing the commercialization of the process. Therefore, intense research activity persists in searching for cost-effective and earth-abundant non-platinum materials with low cathodic overpotential value. Non-noble metals (Ni, Co, Mo, Fe) and their metal oxides, sulfides, phosphides, and some other porous materials, including conductive metal-organic framework and other doped carbon materials, are being explored for their possibilities as cathodic electrocatalysts [5,6,7,8].

Fe is highly suitable as a cathodic electrocatalyst for the HER reaction because of its easy availability, low cost, and low toxicity. Fe³⁺ doped TiO₂ has been hydrothermally prepared by Khan et al. for photocatalytic hydrogen production using visible light [9]. The use of this Fe/TiO₂ composite electrode for HER in an alkaline medium has been reported by Danilov et al. [10]. The composite coating was synthesized by electrodeposition and is outstanding. HER activity was achieved with an overpotential of 470mV at 250 mA cm⁻². TiO₂ plays a dual role in increasing this catalyst's surface area and active sites. A redox couple containing TiO₂ (TiO₂↔Ti₂O₃↔TiOOH) further reinforces the catalytic activity.

The mesoporous structure of TiO₂ nanoparticles supported metal catalysts offer higher stability. Among the anatase and rutile phases of titania, the anatase phase offers a better charge carrier lifetime due to being indirect band gap material. Moreover, the pure anatase phase delivers high surface area and pore volume, leading to better reactant molecule adsorptions to carry out electrocatalytic reactions [11]. The band gap of TiO₂ can be shortened further by doping with both metallic and non-metallic elements, thus positively impacting their photocatalytic and electrocatalytic performance. Doping can also enhance metal-support interactions, leading to increased electrocatalytic properties [12,13]. However, titania's applications are limited by its lower electrical conductivity. One of the major reasons why Fe has not been explored elaborately as an HER catalyst is that 3d transition metals are unstable in acids. However, an acidic medium is generally preferred for HER reactions [14]. This is because, in the acidic medium, the number of protons in the electrolyte is higher, leading to better facilitation of hydrogen generation.

The flaws of TiO₂ and Fe nanoparticles regarding lower electrical conductivity and corrosion in the acidic medium are conventionally overwhelmed by using nanostructured carbon materials (like graphene, amorphous carbon, and multiwalled carbon nanotubes (MWCNTs) [15]. However, the use of Polyaniline (PANI) is not as much discussed in this aspect as the carbon-based materials, although PANI has been regarded as one of the promising candidates for electrocatalytic reactions, and supercapacitor because of its excellent chemical stability, high conductivity, strong adsorption capability of reactants, and also corrosion resistivity towards active metal elements [16,17]. Hence, our present study synthesized a PANI-based composite for HER. But regardless of the above-mentioned unique properties of PANI, HER doesn’t benefit much from PANI alone because of its ill-suited hydrogen adsorption. Recent studies have focused on, improving catalytic activity by optimizing the interfacial coupling between the support and active component to finely tune their electronic states and active sites [18,19]. Here, we tried to increase the efficiency of PANI towards HER by interfacial electronic coupling with Fe-doped TiO₂. The composite Fe-doped TiO₂@PANI has been synthesized by a simple and green in situ polymerization method. The physical properties of the catalyst are investigated by X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The electrochemical performance of the electrocatalyst is evaluated by studying its polarization curve and Tafel plot. The electrostatic impedance spectroscopic (EIS) analysis technique assesses the catalyst's conductivity. The plausible mechanism of HER is also discussed.

2. Materials and Methods

2.1 Materials Used

All the chemicals including titanium tetraisopropoxide (TTIP), Iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), Methanol (CH₃OH), Ammonium peroxodisulfate ((NH₄)₂S₂O₈), Hydrogen peroxide (H₂O₂), Aniline (C₆HNH₂), Acetic acid (CH₃COOH), Nafion and Sulphuric acid (H₂SO₄) purchased from Sigma-Aldrich are of analytical reagent grade purity and were used as received.

2.2 Methods

2.2.1 Synthesis of Fe-Doped TiO₂

Fe-doped TiO₂ nanoparticles were synthesized via a simple co-precipitation method [20] similarly described by Patle et al. 1 M TTIP into distilled water was prepared with continuous stirring for half an hour (solution ‘A’). 0.05 M ferric nitrate in distilled water with continuous stirring for half an hour (solution B). Subsequently, solution B was mixed with solution A, and the solution mixture was vigorously stirred for 2 hours. The precipitate was then filtered and washed with water several times. Thus, obtained white precipitate was mixed with 20 ml H₂O₂ to get a transparent orange solution. The solution was diluted with 10 ml of distilled water until the color changed from orange to yellow. The solution was kept for 2 days for aging. Further, the solution was filtered and washed with distilled water dried at 100^°C in the oven, and calcined at 500^°C for 6 hours.

2.2.2 Preparation of PANI

In a typical procedure, 1.82 mL of aniline [21] monomer was dissolved in a 100 mL reaction solution of 0.4 M acetic acid and 1 M methanol in 200 mL distilled water. 5.71 g ammonium persulphate was similarly dissolved to provide 100 mL solutions. The solutions of aniline and oxidant were cooled at 0-5^°C for 30 mins and then mixed rapidly into a beaker and stirred vigorously for 30 sec. The mixture was left still to react for 10 hours at 0-5^°C. The dark green precipitate was filtered and washed with distilled water and dried in air for 24 hours to obtain PANI.

2.2.3 Synthesis of Fe-Doped TiO₂/PANI

In the preparation of Fe-doped TiO₂/ PANI, the reaction solution of 0.4 M acetic acid and 1 M methanol in 100 mL distilled water was prepared. 0.91 mL of aniline monomer was dissolved in a 50 mL reaction solution, and 5.71 g ammonium persulphate (APS) was similarly dissolved in another 50 mL reaction solution. In the reaction solution of aniline, 0.7 g of Fe-doped TiO₂ was added and stirred for 30 mins. The aniline and APS solutions were cooled at 0-5^°C for 30 mins and then stirred vigorously for 30 sec. The mixture was left still to react for 10 hours at 0-5^°C. The produced was filtered and washed with distilled water and dried in air for 24 hours to obtain Fe-doped TiO₂/PANI (0.45 wt%) nanocomposite. The process was repeated with 0.80 wt% PANI content and termed to Fe-doped TiO₂/PANI (0.80 wt%) nanocomposite.

The notations, Fe-doped TiO₂/PANI and Fe-doped TiO₂@PANI, have been used interchangeably throughout the paper. Also, Fe-doped TiO₂/PANI signifies the PANI content of 0.45 wt%.

2.3 Characterization and Measurement:

To study the effect of Fe and TiO₂ on the structure of PANI nanoparticles, XRD patterns of the samples were recorded on an X-ray diffractometer (Bruker D8-advance) in the 2θ range from (15-80)^°. The samples' FTIR spectra were acquired using a Nicolet Impact-410 IR spectrometer. The surface morphologies of the sample were observed by using SEM (JSM6390LV). All the electrochemical measurements were performed on a Bio-Logic SP-150 potentiostat. The electrochemical behaviors of the samples were studied using a standard three electrodes cell compartment where Pt and Ag/AgCl were used as the counter and the reference electrodes, respectively. The working electrode was prepared by dispersing 6 mg of the synthesized electrocatalyst in 0.1 mL 0.5 wt% Nafion solution and coated onto the ∼0.07 cm² surface area of the glassy carbon electrode (GCE, radius 1.5 mm) and then naturally dried at room temperature. 0.5 M H₂SO₄ aqueous solution was used for all the electrical measurements.

3. Results

To characterize their comparison of XRD patterns of TiO₂ and Fe-doped TiO₂ led to that after doping, there is a slight shift of the peaks towards the lower angle. Crystalline nature, the samples were examined by XRD. Hydrothermally synthesized TiO₂ nanoparticles show reflections at 2θ = 25.43°, 37.76°, 48.14°, 53.95°, 55.07°, 62.76°, 68.96°, 70.26°and 75.15° which belong to (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of titania [22], respectively in pure anatase phase, no rutile peaks are detected (Figure 1(a)). Comparison of the XRD patterns of TiO₂ and Fe-doped TiO₂ led to a slight shift of the peaks towards the lower angle. The shifting signifies the difference in the radii of the lattice and dopant atom. However, the absence of any peaks other than the anatase phase in the XRD pattern confirms successful doping. In the obtained spectra of PANI (Figure 1(b)), two prominent are observed, one at peaks 2θ = 20.4° for the (020) plane and another at 2θ = 25.6° for the (200) plane [23]. After incorporating Fe-doped TiO₂ into PANI, a prominent peak was observed at 2θ = 25.43°. The intensity of this peak was distinguishable due to the merging of the (200) plane of PANI with the (101) plane of TiO₂.

Figure 1 XRD pattern of (a) Comparison of Fe doped TiO₂ and TiO₂, (b) Fe doped TiO₂@PANI, Fe doped TiO₂ and PANI.

The FTIR spectra of pure PANI and Fe-doped TiO₂@PANI are shown in Figure 2. The broad peak at 3437 cm^-1 for both PANI and Fe-doped TiO₂@PANI is observed to be the N-H stretching vibration of the amino group of polyanilines [24]. The peak at 2926 cm^-1 is assigned to the C-H stretching frequency. For the C-N stretching frequency, the peak observed is at 1304 cm^-1, and the peak at 1572 cm^-1 represents the benzenoid ring. The C=C vibration of the aromatic ring indicating the polymer chain was observed at the peak 1486 cm^-1 [25,26,27]. The Fe-doped TiO₂/PANI spectrum shows a significant absorption peak at ~1025 cm⁻¹ due to the Ti-O-C bond confirming the covalent linkage of Fe-doped TiO₂ particle to the PANI [28].

Figure 2 FTIR spectra of pure PANI and Fe doped TiO₂@PANI.

SEM image of Fe-doped TiO₂@PANI shows rough surfaces compared to that of PANI. However, no major change has been observed, implying that the major component is PANI. EDX analysis suggests the presence of C, N, O, Ti, and Fe elements in the expected ratios with negligible impurities. In Figures 3 (d) and 3 (e), the TEM images show a nearly homogeneous dispersion of metal oxide nanoparticles on PANI without any agglomeration. HRTEM image Figure 3(f) exhibits the separation of lattice fringes by 0.35 nm, which corresponds to the (002)) plane of PANI. A d spacing of 0.34 nm, corresponding to the significant (101) crystal plane of anatase phase, which is in good agreement with the p-XRD data (Figure 1(b)) and verifies that Fe doped TiO₂ particles are well supported on PANI.

Figure 3 SEM images of (a) pure PANI, (b) Fe doped TiO₂@PANI, (c) EDX spectra of Fe doped TiO₂@PANI, (d, e) TEM images of Fe doped TiO₂@PANI at various resolutions shows distribution of Fe doped TiO₂ nanoparticles on PANI and (f) HRTEM of Fe doped TiO₂@PANI with lattice fringes corresponding to d (101) plane and d (200) planes.

The elemental valence state and the surface chemical composition of the Fe-doped TiO₂@PANI catalyst were characterized by X-ray photoelectron spectroscopy (XPS) analysis. From the high-resolution survey spectra (Figure 4a), a significant peak was observed in C 1s orbit upon fitting at 283 eV due to C-N functional group, another strong peak at 284.5eV and 285.7 eV corresponds to the C-C/C=C functional groups and oxidation developed C=O functional group. XPS spectrum of N 1s exhibited three significant peaks representing the presence of nitrogen in three different states the quinoid amine at 398.3 eV, the benzenoid amine at 399.5 eV, and the diminished nitrogen cationic radical (N^+•) at 401 eV. Figure 4(c) reveals the Ti 2p spectrum split into two characteristic bands of Ti 2p_3/2 and Ti 2p_1/2 at the binding energy value of 458.7 eV and 464 eV, respectively reflected the presence of oxidized Ti⁴⁺ species [22] and the binding energy of TiO₂ was not affected by dopant concentration. O 1s spectrum shows two characteristic peaks at 529.6 eV and 531.2 eV due to the Ti-O bond and another peak at 533 eV due to several surface species, including hydroxyls, chemisorbed oxygen [29]. In Figure 4(e), the peak observed for Fe 2p confirms the presence of Ti–O–Fe bonds due to the substitution of Ti⁴⁺ by Fe³⁺ [22].

Figure 4 High resolution XPS spectra of Fe doped TiO₂@PANI a) C 1s, b) N 1s, c) Ti 2p, d) O 1s and e) Fe 2p.

4. Discussion

4.1 Electrocatalytic Activity Towards HER:

The hydrogen evolution activity of Fe-doped TiO₂@PANI composite was examined using a three-electrode set-up in 0.5 M H₂SO₄ solution at room temperature. The polarization curves (J-V plots) obtained from linear sweep voltammetry (LSV) measurements with a scan rate of 50 mV sec⁻¹ were used to determine the catalytic activity. In this study, all the potentials were calibrated with a silver/silver chloride electrode (Ag/AgCl) and then converted to a reversible hydrogen electrode (RHE) using the equation E (RHE) = E(AgCl) + 0.1976 V.

From the non-zero cathodic current measurements in Figure 5(a), the onset potential for Fe-doped TiO₂@PANI with 0.45 wt% was calculated as -0.180 V, which is noticeably better than that of TiO₂@PANI (-0.207 V), Fe doped TiO₂@PANI with 0.80 wt% (-0.227 V) and PANI (-0.360 V) with same catalyst loading. It is observed that Fe-doped TiO₂@PANI with 0.45 wt% requires a relatively smaller overpotential (η₁₀) of -0.285 V vs. RHE to achieve the current benchmark density of 10 mA cm^-2 than that of TiO₂@PANI (-0.311 V), Fe doped TiO₂@PANI with 0.80 wt% (-0.331 V) and PANI (-0.498 V). This boost comes from the strong contact of the Fe-doped TiO₂ particles with the PANI. DFT study reveals that the interplay between N sites of the PANI and metal oxides substrate leads to a favorable electronic structure that optimizes the energy for hydrogen evolution on PANI [30]. According to the Sabatier principle, a good HER catalyst's H* adsorption-free energy (ΔG_H*) is expected to have close-to-zero ΔG_H [31]. Lower ΔG_H results in very strong binding of the atomic hydrogen and hinders the desorption of H₂, while higher ΔG_H prevents the binding of atomic hydrogen on the catalyst surfaces. Thus, the reaction becomes slow unless there is a proper balance between these two conditions. The projected density of states (DOS) study confirmed that PANI has a ΔG_H* of −1.47 eV, implying strong hydrogen adsorption on the N side of PANI [30]. Thus, the desorption of adsorbed hydrogen becomes tough, and HER kinetics turns out to be slow. But upon, the incorporation of metal oxide nanoparticles in PANI due to the electron transfer from the metal atom to the N atom of PANI leads the incoming electrons to be filled in the antibonding states of N. It thus decreases the binding energy of hydrogen with the N atom enhancing the HER kinetics. Furthermore, to examine the role of Fe doping in our catalyst, we studied the change in electronic properties of bare TiO₂ after Fe doping (Figure S2). Dopant Fe modifies the electronic structure of TiO₂ by developing new electronic states within the wide band gap of TiO₂. Thus, electrons easily reach the surface to carry out HER efficiently. The EIS analysis (Figure S2 (a)) also upholds the increased conductivity of Fe-doped TiO₂. In the case of our catalyst also, the electrons from Fe-doped TiO₂ easily reach the surface-active sites upon inducing relatively low electric potential to impel HER efficiently. However, upon increasing PANI content after 0.45 wt%, the composite efficiency towards HER was found to be decreased. This may be due to the increased PANI content covering the active sites and thus deteriorating catalytic activity.

Figure 5 Electrocatalytic performance of various catalysts (a) Polarization curves showing onset potentials for HER (b) Tafel plots, (c) Nyquist plots, (d) time dependence study of the catalyst Fe doped TiO₂@PANI at an overpotential of -200 mV (Vs RHE) in 0.5 M H₂SO₄, (e) cyclic voltammograms at various scan rates from 20 mV s^-1 to 220 mV s^-1 within a non-faradic region, (f) variation of double layer charging current density against scan rate (C_dl values calculated for PANI and Fe doped TiO₂@PANI).

Tafel slopes, an inherent, intrinsic property of the catalysts analyzed the kinetics and the mechanistic pathway of HER. Fe-doped TiO₂@PANI shows a small Tafel slope value of 144 mV dec^-1 (Figure 5(b)). A smaller Tafel slope suggests a faster HER rate and a smaller energy requirement.

Electrocatalytic HER proceeds via two possible routes:

1. 1) The adsorption reaction with the Tafel slope closer to 120 mV dec^-1, i.e., Volmer step.

\[ \mathrm{H}^{+}+\mathrm{e}^{-} \rightarrow \mathrm{H}_{\mathrm{ads}} \tag{1} \]

And

1. 2) either of the following routes
2. a) The electrochemical desorption reaction (Heyrovsky step).

\[ \mathrm{H}_{\mathrm{ads}}+\mathrm{H}^{+}+\mathrm{e}^{-} \rightarrow \mathrm{H}_{2} \tag{2(a)} \]

Or

1. b) The chemical desorption and recombination reaction (Tafel step).

\[ \mathrm{H}_{\mathrm{ads}}+\mathrm{H}_{\mathrm{ads}}=\mathrm{H}_{2} \tag{2(b)} \]

The combination of two of these steps, Volmer-Heyrovsky equation 1 with equation 2 (a) or Volmer-Tafel equation 1 with equation 2 (b) mechanism led to the formation of molecular hydrogen [32]. Tafel slope analysis here ensures that the HER kinetics is likely controlled by the Volmer-Heyrovsky mechanism, and releasing protons on the electrode, i.e., the Volmer step, is the rate-determining step. The plausible mechanism of HER on Fe-doped TiO₂/PANI is as follows:

\[ \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{e}^{-}+\mathrm{N}^{*} \rightarrow \mathrm{NH}_{\mathrm{ads}}+\mathrm{H}_{2} \mathrm{O} \tag{Volmer step} \]

\[ \mathrm{NH}_{\mathrm{ads}}+\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{e}^{-} \rightarrow \mathrm{H}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{N}^{*} \tag{Heyrovsky step} \]

N* represents the active nitrogen sites of PANI and H_ads represents the adsorbed hydrogen.

To study the charge transfer resistance between the electrode-electrolyte interface, electrochemical impedance spectroscopy (EIS) analysis was carried out at a fixed overpotential of -270 mV vs. RHE within the frequency range from 100 MHz to 10 kHz. The charge-transfer resistance (R_ct) values derived from the Nyquist plots (Figure 5(c)) are 753 Ω, 469 Ω, and 335 Ω for PANI, Fe doped TiO₂@PANI (0.8 wt%), and Fe doped TiO₂@PANI (0.45 wt%) respectively. A smaller diameter suggests a quicker charge transfer process meaning better conductivity. The smaller R_ct value of Fe-doped TiO₂@PANI (0.45 wt%), mainly due to the efficient electron transfer from the electrode to the electrolyte, suggests the better conductivity of the catalyst validating its faster HER kinetics [33].

The durability of the catalyst was checked by performing chronoamperometric (CA) analysis for ~7 h at -200 mV (vs. RHE) overpotential (Figure 5(d)). The analysis graph fluctuated due to dehydrogenation via bubble generation and restoration of current density after the bubble detachment process. However, the catalyst maintained its integrity in catalytic efficiency towards the end. The durability test represents the applicability of the composite in HER in such an acidic environment for a long period.

Regarding the remarkable catalytic performance of Fe-doped TiO₂@PANI, the voltammetric analysis was carried out within the potential range from +0.2 V to +0.43 V with different scan rates where no faradic reaction occurred to discern the electrochemical active surface area (ECSA) (Figure 5(e)). Higher ECSA value signifies more catalytically active sites. ECSA was calculated from the electrochemical double-layer capacitance (C_dl) that was obtained by fitting the values of the capacitive current (i) and scanning rate (ν) in equation 3.

\[ i=v C_{d l} \tag{3} \]

C_dl values calculated for Fe-doped TiO₂@PANI and PANI are shown in Figure 5(f). Furthermore, the roughness factor (R_f) was calculated from the C_dl values, which is the ratio of the C_dl on 20 μF cm⁻² (capacitive surface of a smooth electrode) [34,35]. Calculated C_dl, R_f and ECSA values for the catalysts are given in Table 1. PANI has a specific role in the homogeneous distribution of Fe-doped TiO₂ nanoparticles and thus increases active surface sites for better HER [17]. From these values, it can be proposed that Fe-doped TiO₂@PANI has an advantage in active surface area (ECSA) and active sites over PANI to improve electrocatalytic HER activity.

Table 1 C_dl, R_f and ECSA values obtained for PANI and Fe doped TiO₂/PANI.

5. Conclusions

In summary, this study successfully applies environmentally friendly and cost-effective catalyst Fe doped TiO₂@PANI towards HER synthesized via a green method. The composite requires a comparatively small onset potential of 0.180 V than PANI (0.360 V). We have tried to upgrade the catalytic efficiency of PANI by interfacial electronic coupling with Fe-doped TiO₂. The efficiency improvement is solely attributed to the occupancy of the antibonding states, leading to a weaker interaction between adsorbate hydrogen and catalyst surface. However, better conductivity and improved surface area due to PANI are other factors for better catalytic performance.

Acknowledgments

The author would like to thank the Sophisticated Analytical Instrumentation Centre (SAIC), Tezpur University, India for providing analytical support.

Author Contributions

Suman Lahkar is the first author in this paper, her contributions include experimental design and operation, electrochemical measurements, and the writing of the paper. Richa Brahma helps to carry out some part of the characterization. Swapan Kumar Dolui supervises the entire work.

Competing Interests

The authors have declared that no competing interests exist.

Additional Materials

The following additional materials are uploaded at the page of this paper.

1. Figure S1: XPS survey spectra of Fe doped TiO₂@PANI.

2. Figure S2: Comparison of electrocatalytic performance of TiO₂ and Fe doped TiO₂ (a) Polarization curves showing onset potentials for HER, (b) Nyquist impedance.